T cell dysfunction upon ongoing antigen exposure is a cardinal feature of chronic infections and cancer in animals and humans, leading to T cell impairment and failure to eliminate virus or tumor cells. These defective T cell responses are thought to play a major role in HIV infection and progression to AIDS. Previous work performed at our center and by other investigators of this program grant has shown in both HIV infection and animal models that the Programmed Death-1 (PD-1) pathway is a crucial mediator of T cell dysfunction, and that blocking this pathway can reinvigorate CD8 (CTL) and CD4 T cell responses. PD-1, its ligands PD-L1 and PD-L2, and the PD-L1 ligand CD80 participate in a network of pathways that modulate T cell function The goal of this proposal is to elucidate the role of PD-1 and its ligands in AIDS-related T cell dysfunction; successful achievement of these aims will help guide the design of new clinical interventions to reverse immune failure in HIV-infected subjects. In Aim 1, building on previous results correlating PD-1 expression by HIV-specific CD8 and CD4 T cells with disease progression, we will compare the expression and functional impact of PD-1 on T cells from subjects with acute infection, chronic infection, or spontaneous viral control (elite controllers). Using state-of-the-art imaging techniques from Core C, we will determine the role of PD-1 in modulating exocytosis of cytolytic granules by HIV-specific CTL. We will also investigate PD-1 expression and function in elite controllers who present a polymorphism in the PD-1 gene or in PD-1 regulatory elements, as identified in Projects 2 and 4. In Aim 2, we will determine qualitative differences between HIV-specific CTL expanded in the presence or absence of PD-1 blockade, with respect to immunodominance, cytokine production, and cytotoxicity. We will determine the roles of PD- 1 expression and TCR avidity in the response of HIV-specific CTL to PD-1 blockade. In Aim 3, we will determine the relative contributions of multiple pathways engaged by PD-1, PD-L1, and CD80 a key antiviral CTL function, namely the ability to suppress HIV replication in vitro. Starting from established methods to measure viral suppression, we will systematically knock down or block various PD-L1-mediated pathways to determine the role of each pathway in antiviral function. We will determine the relative roles of CTL proliferation, survival, cytokine secretion, and killing of infected targets in PD-L1 modulated antiviral activity. Relevance to public health: The immune system of HIV-infected people includes cells (T cells) that recognize HIV, but fail to block the virus and to prevent progression to AIDS. Our previous studies have shown that these T cells express a protein, called PD-1, which inhibits their function. We are studying the role of PD-1 in preventing proper T cell responses, and looking for ways to rescue the ability of T cells to control HIV.